Liquid gauging apparatus and remote sensor interrogation

ABSTRACT

A liquid gauging system for a liquid container, the system comprising sensor means for producing a first electromagnetic signal that corresponds to liquid quantity in the container, and remote interrogation means for receiving the first electromagnetic signal and producing a system output that corresponds to the liquid quantity; the sensor means being energized by a second electromagnetic signal transmitted by the remote interrogation means.

This is a division of pending application Ser. No. 08/069,263 filed May28, 1993, now U.S. Pat. No. 5,399,875.

BACKGROUND OF THE INVENTION

The invention relates to apparatus and methods for liquid gauging, andmore particularly to apparatus and methods for remotely interrogatingliquid quantity sensors.

Many types of liquid quantity and level sensors are known, includingcapacitive sensors, resistive sensors, acoustic sensors and so forth.Passive sensors generally operate on the basis of a sensor element thatexhibits a parameter, e.g. capacitance, that varies with the liquidlevel. Active sensors such as acoustic sensors operate on the basis ofproducing a signal, e.g. an acoustic pulse, that can be used to detectthe liquid level by parametric analysis such as echo ranging.

Such systems further include an electronic circuit that detects theparametric value of interest and converts that value to a signal thatcorresponds to the liquid level.

A common application for such liquid level sensors is for fuel level andquantity detection in aircraft fuel tanks. However, due to the volatilenature of fuel, it is desirable to minimize the connection of electricalenergy to the sensors which may be disposed in the fuel. It is furtherdesirable to minimize the amount of electrical energy stored in thesensors or used by the sensors.

A known approach for minimizing the coupling of electrical energy into afuel tank is described in U.S. Pat. No. 4,963,729, issued to Spillman etal., and commonly owned by the assignee of the present invention. Inthis system, optical energy is coupled to the sensors via optic fibers.This optical energy is then converted to electrical energy forenergizing the sensors. The sensors detect the liquid level and thentransmit another optical signal back to a detector via the optic fibers.The detector then converts the second optical signal into an output thatcorresponds to liquid level in the tank.

For aircraft applications, on board readings often need to be verifiedby ground crews, either during routine turn around or to confirm anerror reading. The optical fiber link to the internal sensor in theabove system prevents as a practical matter interrogation of the sensorby ground crews, other than via the same optic fiber link which may infact be the cause of a fault reading.

A commonly used fuel level sensor in commercial aircraft particularly isa dripstick sensor, which is used as a backup fuel gauging apparatus tothe on-board electronic fuel level sensors. For example, dripstickverification may be needed when a refueling truck gauge disagrees withthe aircraft fuel gauge, if the on-board fuel gauges appear to beinaccurate or inoperative, or simply by request of the flight crew,among other possible reasons.

The dripstick includes a linear body that extends vertically into thefuel tanks. Often there is a plurality of such dripsticks in each wingof the aircraft. A magnetic float is disposed on the dripstick body likea collar that floats at the fuel surface. The dripstick is read by theground crew by manually withdrawing the dripstick from the wing until amagnetic tip at the upper end of the sensor body engages the float. Theoperator can feel the resistance of the tip against the magnetic floatand stop pulling on the dripstick. The dripstick body includes a seriesof markings which visually indicate to the operator the fuel level basedon how far the dripstick was withdrawn from the tank. Although dripstickdesigns may vary somewhat, the basic operation of manual access andvisual interrogation is the same for the ground crew.

Various problems are associated with using the conventional dripsticks,especially the time involved for the ground crew to walk around to allthe sensors and manually/visually determine the readings. The mechanicclimbs a ladder or uses a lifting device to gain close access to theunderside of the wing, withdraws the dripstick until the engagement isdetected, records the reading and then replaces the dripstick into thetank. Dripstick design is further complicated by the need for minimalfuel leakage. This entire process must be repeated for each sensor,which adds substantially to the refueling operation and turnaround timefor aircraft flight readiness.

Commercial air carriers have long identified the need for adripstick-like backup system, but one that is easier to use. A systemthat can be interrogated from the ground would eliminate the need forlifting equipment and provide easier reading of difficult accessdripsticks. Overall reduction in refueling and fuel verification delayscould then be realized.

The need exists therefore for simple and reliable apparatus and methodsto interrogate liquid gauging sensors from a remote, preferably groundlevel, location without coupling electrical energy into a volatilecontainer. Such an arrangement should also be compatible with currentdripstick sensor configurations if desired for a particular application.

SUMMARY OF THE INVENTION

Accordingly, the invention contemplates a liquid gauging system for usewith a liquid container, the system comprising sensor means forproducing a first electromagnetic signal that corresponds to liquidquantity and/or level in the container, and remote interrogation meansfor receiving the first electromagnetic signal and producing an outputthat corresponds to the liquid quantity; the sensor means beingenergized by a second electromagnetic signal transmitted by the remoteinterrogation means.

The invention also contemplates the methods for remotely sensing liquidquantity in a container as embodied in such apparatus, as well as amethod for remotely detecting liquid quantity and/or level in acontainer comprising the steps of:

a. producing a first electromagnetic signal and transmitting the firstelectromagnetic signal to a sensor that produces an output thatcorresponds to liquid quantity in the container;

b. using the first electromagnetic signal to energize the sensor;

c. producing a second electromagnetic signal that corresponds to liquidquantity in the container; and

d. transmitting the second electromagnetic signal to a remote detectorfor converting the second electromagnetic signal into an output thatcorresponds to liquid quantity in the container.

These and other aspects and advantages of the present invention will bereadily understood and appreciated by those skilled in the art from thefollowing detailed description of the preferred embodiments with thebest mode contemplated for practicing the invention in view of theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a simplified representation of a liquid quantity gaugingsystem according to the invention, such system being generally shown inan exemplary manner for use with aircraft fuel gauging;

FIG. 1B is an enlarged simplified view of an optics arrangement for theexemplary use depicted in FIG. 1A;

FIG. 2 is a system functional block diagram for a sensor interrogationsystem and method that uses a remote hand held interrogation unit andelectronic control circuit according to a preferred embodiment of theinvention;

FIG. 3 is a simplified flow chart for operational functions of thesystem illustrated in FIG. 2;

FIG. 4 is a partial cross-sectional view of a dripstick sensor suitablefor use with the present invention;

FIG. 5 is a schematic diagram of an energy conversion and controlcircuit suitable for use with the present invention, and FIG. 5A is aschematic of an alternative and simpler circuit suitable for use in manyapplications; FIG. 6A, 6B and 6C is a schematic diagram of sensorinterface and encoder circuits suitable for use with the invention, andFIG. 6D shows another suitable embodiment for an encoder circuit;

FIG. 7 is a timing diagram showing representative pulses produced by anencoder circuit suitable for use with the present invention; and

FIG. 8 is a schematic diagram of an LED/Driver circuit for the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

With reference to FIG. 1A, aircraft, particularly large commercialaircraft, have a plurality of fuel tanks 10 internal to the wingstructures. Although the invention is described herein with particularreference to commercial aircraft, this is for purposes of explanationonly and should not be construed in a limiting sense. Those skilled inthe an will readily appreciate that the invention can conveniently beused with any aircraft or other vehicles or structures that have liquidcontainers. The invention also is not limited to fuel tank containers ofvolatile fuel, but can be used for sensing liquid quantity and levels ofmany fluid types in virtually any container. The particular sensor usedfor detecting the fuel level/quantity, such as, for example, amagnetoresistive dripstick as described in an exemplary manner herein,is also a matter of design choice. Those skilled in the an will readilyappreciate that the advantages and benefits of the invention can berealized using any sensor that produces an electrical output or producesan output or exhibits a characteristic that can be interpreted byelectronic circuits including capacitive, resistance, acoustic and so onto name just a few. The terms "liquid level" and "liquid quantity" asused herein are intended to be understood in their broadest sense andare essentially interchangeable terms. As is well known, liquid leveldata can easily be converted to liquid quantity data, and vice-versa,when the tank or container dimensional characteristics are known. Theinvention is directed in a broader sense to apparatus and methods forremotely interrogating sensors that produce outputs; and is especiallyuseful with sensors that produce outputs corresponding to liquidquantity and/or level in the fluid container.

In the embodiment described herein, each fuel tank or liquid container10 includes one or more quantity/level sensors 12. For example, largecommercial aircraft such as the Boeing 747 may use nineteen suchsensors. Other applications and containers may use different numbers ofsensors or only one sensor.

Each sensor 12 can be of any convenient design that is preferablyelectrically interrogated. In other words, the particular sensorselected is optional for the designer, but preferably will be a sensorthat produces (or is connected to a transducer that produces) anelectrical output signal that corresponds to the quantity/level ofliquid in the container. One example of many of a commonly used sensoris a capacitive sensor in which a capacitive element changes capacitancevalue in relation to the percent immersion of the element in the liquid.Such a sensor is described, for example, in U.S. Pat. No. 4,841,227issued to Maier and commonly owned by the assignee of the presentinvention, the entire disclosure of which is fully incorporated hereinby reference.

The present invention is particularly well suited for use with adripstick sensor 12 that is commonly used to mechanically read fuellevels in aircraft fuel tanks. In a conventional dripstick design, thedripstick sensor 12 includes an elongated linear body 14 that isvertically mounted in the fuel tank. For bottom mounted dripsticks, amagnetic or ferrous tip 16 is placed at the upper end of the dripstick.A magnetic float 20 in the form of a collar is slidably placed aroundthe elongate body and floats on or near the fuel surface 18. When theground crew pulls the dripstick body out of the tank, the mechanic canphysically sense the position when the tip 16 engages the floating ring20. Calibrated markings on the dripstick body then provide a visualreading of the fuel level and hence volume in the tank, whichinformation can be used as a confirmation of on-board fuel levelreadings.

The present invention provides a simple and accurate way to interrogatesuch a dripstick sensor without the need to mechanically withdraw thedripstick from the fuel tank (as will be more fully explainedhereinafter with respect to a sensor such as illustrated in FIG. 4.) Themodified dripstick described herein, however, can also be dualconfigured in a conventional manner for manual withdrawal in case aredundant manual backup system is desired.

More generally, though, the invention contemplates apparatus and methodsfor interrogating many different types of sensors using a remote handheld unit.

As illustrated in greater detail in FIG. 1B, the sensor 12 (which inthis case will be the modified dripstick described hereinafter) iselectrically coupled to a sensor electronic control circuit 40. Thecontrol circuit 40 may be disposed in a unit that also is used to mountthe sensor 12 in the tank. The control circuit 40 includes a circuitthat receives electromagnetic energy 32 (FIG. 1A) from a remote controlpreferably hand held unit 34 and converts this energy to electricalpower for the control circuit 40. Thus, on-board electrical power doesnot need to be connected into the fuel tank, thereby reducing electricalenergy in the tank.

The control circuit 40 further includes a circuit that detects thevariable parameter of the sensor 12 (e.g. the resistance value orcapacitance value as a function of percent immersion) and emitselectromagnetic energy 36 having a characteristic that is modulated inrelation to the fluid quantity and/or level detected by the sensor 12.For example, the control circuit 40 may emit electromagnetic pulseshaving a duty cycle or time base modulation that corresponds to theliquid quantity detected. Other modulation and encoding schemes can, ofcourse, be used. The encoded data may further include information inaddition to the liquid quantity. For example, the sensor control circuit40 can be used to encode information such as the type of sensor used,the type of aircraft it is installed in, which sensor corresponds to theparticular circuit 40 and so forth. Sensors that detect fuelcharacteristics other than quantity can also be interrogated if desired.Each sensor control circuit 40 preferably also emits a digital code orprotocol so that the data signals can be properly detected andidentified.

According to an important aspect of the present invention, the sensorcontrol circuit 40 is remotely energized and, in combination with theremote control unit 34, interrogates the sensor 12. Preferably, theremote unit 34 is a small portable hand held unit easily used by groundcrews. When activated, the hand held unit transmits electromagneticenergy 32 to the sensor electronics 40 wherein it is converted andstored as useful electrical energy. This stored electrical energyprovides the power needed to energize the sensor 12 and also to energizethe circuitry needed to transmit the sensor data and identification backto the hand held unit 34 via the modulated electromagnetic energy 36. Adetector circuit in the hand held unit detects the modulated beam 36,demodulates the signal to produce electrical signals containing thedesired sensor information and fuel data, and if desired presents theinformation on a visual display 38 or in another output form.

A system level approach to such apparatus is illustrated in FIGS. 2 and3. A sensor control circuit is generally indicated with the numeral 40.The sensor control circuit 40 includes one or more signal control lines42 that connect the control circuit 40 to the sensor 12 (oralternatively a plurality of sensors when the circuit 40 is configuredto operate in a multiplexed mode.) The signal control lines 42 may, forexample, be used to supply electrical power to a sensor or to receive anoutput signal from a sensor, or simply to connect a sensor element (suchas a variable component) into a detector loop within the circuit 40. Thespecific nature of the interface of the sensor 12 to the circuit 40 willbe determined by the type of sensor being used. In the describedembodiment of this invention, the sensor output is essentially a voltageor current that corresponds to the resistance value of the dripstickmagnetoresistive element. The sensor 12 output could just as easily bethe actual resistance itself of the sensor connected by the leads 42 toan impedance sensitive detector in the control circuit 40. As anotherexample, the circuit 40 could be optically coupled to the sensor 12, ormagnetically coupled to the sensor. A direct hard wired connection isnot essential depending on the type of sensor being interrogated and themethod of interrogation selected. The important aspect is that thecircuit 40 is configured to interrogate or determine the sensor 12output condition after electrical power has been input to the circuit 40by operation of the hand held unit 34, which condition corresponds tothe fuel level or quantity in the tank. Until electrical energy isdelivered to the circuit 40 via activation of the remote unit 34, thesensor 12 and the circuit 40 are preferably and substantiallyde-energized.

As further represented in FIG. 2, the remote unit 34 is remotelyconnected to the control circuit 40 via the transmitted and receivedelectromagnetic energy signals 32,36. The remote unit 34 circuitry isappropriately configured to transmit electromagnetic energy, such as forexample, photoelectric energy such as infrared light (32) to the controlcircuit 40, and to receive photoelectric energy such as infrared light(36) from the control circuit 40. The remote control unit 34 isoptically linked through air to the control circuit via the lighttransmission alone, without the use of optic fibers or other lightconduits, or electrical connections thereby maximizing the flexibility,portability and simplicity of use of the hand held unit 34. Thus, theterm "remote" as used herein refers to electromagnetic coupling withoutthe use of optical or electrical connections between the remote unit andthe sensor electronics.

The control circuit 40 may be designed to perform any number offunctions depending on the number and type of sensors being interrogatedwith each circuit 40. However, generally the control circuit 40 willinclude at least the following functional circuits. An energy receiver,such as a photodiode array 44 is used to receive the electromagneticenergy transmitted from the hand held unit 34, and to convert thatenergy into useful electrical energy. The type of devices used in thearray 44 will, of course, depend on the spectral content of theelectromagnetic energy used to link the hand held unit 34 to the controlcircuit 40. Although infrared light is one of the preferred options itcertainly is not the only option available. Broadband radiation such asfrom white tungsten lamps, radio frequency waves and other spectralbands can be used for the electromagnetic energy. Whatever bandwidth isselected will determine the specific detector 44 used, which must beresponsive within the spectral band of the electromagnetic energytransmitted to the control circuit.

In the described example, the diode array 44 produces electrical energyin response to the received light 32. A power storage circuit 46receives and stores this electrical energy for use in powering thesensor 12 and the circuit 40 under the control of a power controlsequencer circuit 48. The sequencer circuit 48 is designed tosequentially control the application of power to the sensor 12 (orsensors in a multiplexed design) via a sensor interface circuit 50.Depending on the overall complexity of the sensor 12 and the circuit 40,the interface circuit 50 may be as simple as a switch, a voltage levelshifter, or more involved such as a multiplexer, transducer and so on.Whatever functions are selected for the circuit 40 will be determinedprimarily by the amount of electrical energy that can be stored andretrieved from the storage circuit 46, and how much of a load thecircuit 40 applies to the storage circuit.

The sensor output information is input to an encoder circuit 52 that inturn is connected to an electromagnetic source 54, such as an LED arrayand driver circuit. In general, the overall electrical load of thecircuit 40 is primarily dependent on the power consumption in the LEDcircuit, especially if the circuit 40 is expected to transmit the sensordata over a long distance to the hand held unit 34. Therefore, sensordata is preferably encoded in a digital manner so that the source 54 canbe pulsed rather than continuously operated.

The hand held unit 34 also includes an electromagnetic energy source 56which for convenience may be an LED and associated driver circuitsimilar to the control circuit light source circuit 54. However, theremote hand held unit could transmit electromagnetic energy for purposesof energizing the sensor using a different spectral band than is usedfor the encoded energy transmitted back to the remote unit. A detectorcircuit 58 is used to receive the encoded signals from the sensorcontrol circuit 40 and to convert those pulses into electrical signalsthat can be processed and interpreted. The detector circuit 58 can be asimple photodiode or photodiode array or other transducer, and mayinclude signal conditioning circuits for amplification, filtering andother signal processing functions well known to those skilled in theart.

The detector 58 signals are input to a central processing circuit 60which for convenience may be a control circuit utilizing amicroprocessor or similar controller. Discrete logic and signalprocessing, of course, could also be used. The controller 60 operates apower control circuit 62 that switches the light source 56 on and off atappropriate times. The source 56 is used, as stated, to supplyelectrical energy to the sensor circuit 40. The source 56 can bedesigned to be operated continuously during a sensor interrogationoperation, or can be turned off when the hand held unit is ready toreceive the encoded light pulses.

The controller 60 is programmed in a conventional manner to interpretthe received data from the sensor 12 and convert that data into anoutput, such as the visual display 38 used to display fuel level,quantity, sensor identification, plane identification or any other datatransmitted back to the remote unit 34. The controller 60 also monitorsan operator interface circuit 64, which may be realized in the form ofan alphanumeric keypad optionally provided with special function keysand so forth as is well known. The operator interface can be used, forexample, to control actuation of the light source 56, as well as toinput control data such as the type of plane, which sensor is beingaccessed, and so on when such information is not transmitted by theon-board sensor.

With reference now to FIG. 3, a suitable overall system functional flowdiagram for the apparatus illustrated in FIG. 2 is provided. Again, thisflow diagram is only intended to be exemplary, the actual functions andsteps being performed by the fuel gauging system being ultimatelydetermined by the complexity and data requirements specified for aparticular application.

At step 70, the controller 60 turns on the energy source 56 asinstructed by the mechanic by activation of the hand held unit 34. Thesource 56 transmits electromagnetic energy towards one of the dripsticksensors by means of the mechanic aiming the hand held unit at theassociated sensor control circuit 40. The detector array 44 is exposedto the incident energy, such as through an optical window (not shown)that seals the electronics from the environment exterior to the wing ortank. The control circuit 40 converts the received electromagneticenergy to electrical energy and interrogates the sensor 12. The sensoroutput is then encoded and transmitted back to the hand held unit viathe modulated source 54. Thus, at step 72 the detector circuit 58receives the encoded signal 36, processes the signals as required, andat step 74 the controller 60 determines the pulse timing characteristicsto interpret the sensor 12 outputs. The controller 60 also decodes thedata signals with the encoded information such as the dripstick number,aircraft type and so on. At step 76 the controller, by means ofappropriate lookup tables and algorithms converts the decoded signalsinto the desired information such as fuel volume, level, sensor numberand so on, and at step 78 displays the requested information to themechanic via the display 38, which may be a visual display, printeddata, recorded data and so forth.

With reference next to FIG. 4, I show a dripstick sensor that has beenmodified for use with the remote interrogation or direct interrogationby on-board electronics approaches of the present invention. Forclarity, FIG. 4 only illustrates the operative portion of the sensor inaccordance with my invention that detects the fuel height in the tank.Other parts of the dripstick can be conventional in design and are wellknown. The sensor 12 includes the elongated member 14 which may be, forexample, an aluminum tube 80. Within the tube 80 is a precision wirewound resistor 82 which extends substantially along the entire portionof the tube 80 used for detecting the fuel height or level. The float 20is disposed around the tube 80 as a collar, and retains an actuationmagnet 84. As the float moves up and down in relation to the fuel level18, the magnet 84 causes spring fingers 86 that are part of a conductivestrip 88 to contact the wire resistor 82. A magnetic piece 89 may beprovided to insure the fingers 86 return out of contact from theresistor 82 when the float is not aligned with the fingers. One end ofthe resistor 82 serves as a sensor terminal 90 and the conductive strip88 serves as another sensor terminal 92. Thus, the resistance betweenthe terminals 90,92 directly corresponds to the percent immersion of thetube 80 in the fuel, or stated another way, directly corresponds to thefuel level/quantity in the tank. An insulative layer 94 can be providedbetween the tube wall 96 and the internal parts of the magnetoresistivesensor to prevent the tube 80 from electrically short circuiting theresistor 82. This particular sensor 12 can easily be interrogated bysimply applying a voltage or current across the terminals 90,92 anddetermining the resistance value. A particular advantage of thismodified dripstick sensor is that it can also be used manually by theground crew for verifying the fuel level readings as with a conventionaldripstick. Thus, the modified dripstick can be used as a direct fieldreplacement for conventional dripsticks. Also, the magnetoresistiveelement can advantageously be hardwired to the on-board electronics ifsuch monitoring of the dripstick readout is desired.

In FIG. 5, I show one example of a circuit that can conveniently be usedfor energy conversion and power control in the sensor control circuit 40with exemplary component values being provided. In this particulararrangement, the detector array 44 is positioned near an optional lens100 that collimates light energy 32 received from the remote hand heldunit 34 through a window 102 which may be flush mounted with the wingunderside. The array 44 preferably includes a plurality of photocells104 that convert incident light energy into voltage and current. Thecells could be used in the photovoltaic, photocurrent or both modes. Thenumber of photocells 104 used will be determined by the characteristicsof the photocell used, as well as the voltage and charging requirementsof the overall control circuit as dictated by the circuit's powerrequirements needed to interrogate the sensor 12 and transmit encodedlight signals back to the hand held unit 34.

The photocells 104 are connected to a circuit charge storage capacitor106 through a rectifying diode 108. The array 44 is also connected to aLED/Driver (54) storage capacitor 110 through a second rectifying diode112. Separate storage capacitors are preferred for the LED and controlcircuit functions due to their respective load effects on the overallcharging requirements. For example, the circuit storage capacitor 106can be smaller in many applications and thus charged quickly for fastaccess to the sensor 12. Also, by having the LEDs operate from aseparate capacitor, the load effects of the LEDs will have lessinfluence on the operation of the control circuit components used tointerrogate and encode the sensor 12 data. The LED capacitor 11 stores aDC supply voltage V³⁰ 114 at an output node 116, which is connected tothe LED/Driver circuit 54 (FIG. 2). Note that in the preferredembodiment, the DC supply to the LED/Driver 54 is maintained for as longas the capacitor is charged by light received from the remote unit 34,or until the capacitor is discharged by operation of the LED circuit 54.The circuit storage capacitor 106 stores a DC supply voltage thatappears at node 118 with respect to the common return 120. A powerswitch circuit 122 connects the stored energy from the capacitor 106 toa V⁺ terminal and V⁻ terminal 126 which are connected to the sensorinterface circuit 50 (FIG. 2). The circuit 122 operates in aconventional manner to switch on the output transistor 128 when thevoltage stored on the capacitor 106 reaches about +5 VDC, and switchesthe transistor 128 off when the capacitor 106 discharges to about +2.2VDC. The switch circuit 122 provides a low impedance output for thecircuit voltage supply, and also prevents operation of the controlcircuitry until sufficient energy is stored in the capacitor 106 toassure accurate data can be transmitted back to the remote unit 34.

In FIG. 5A I show another and simpler circuit that can be used forenergy conversion and storage when isolated LED and control circuitsupplies are not needed (with like components being given like referencenumerals followed by a prime'). In this circuit, the detector array 44'converts the electromagnetic energy into electrical energy that isstored in the main storage capacitor 106' via a rectifying diode 108'.When the stored voltage reaches a predetermined threshold, an outputtransistor 128' switches on and the supply voltage appears at the V⁺ andV⁻ terminals 124', 120' and is connected to the LED/Driver circuit 54'and the sensor interface circuit 50'.

Referring next to FIG. 6A, 6B and 6C show a set of circuits (A, B and C)that can conveniently be used to detect the dripstick sensor 12 outputand to encode the output for transmission to the remote control unit 34.The V⁺ and V⁻ supplies shown in FIG. 6 are connected to the output nodes124,120 of the supply circuit in FIG. 5. The top circuit in FIG. 6 is asimple voltage scaling circuit that connects the V⁺ and V⁻⁻ suppliesacross a precision reference resistor 140. A reference resistor is usedin this case because the sensor 12 of this example is a magnetoresistivesensor having a resistance value that is the parameter of interest. Thevoltage across the reference resistor 140 produces a reference currentthat is converted into a negative reference voltage, -V_(ref), by aninverting amplifier 142. A second inverting amplifier 144 configured forunity gain produces a positive reference voltage, +V_(ref).

The middle circuit in FIG. 6 realizes the sensor interface circuit 50(FIG. 2) and is used to transduce the sensor 12 output into a usefulelectrical signal. The V⁺ and V⁻ supplies are connected across thesensor resistance via the signal lines 42 (FIGS. 2 and 4). This producesa sensor current, I_(sensor), which is converted to a voltage,V_(sensor), by another amplifier 146.

The voltages +V_(ref), -V_(sensor) and -V_(ref) are input to the lowercircuit in FIG. 6, which realizes the encoder circuit 52 (FIG. 2). Thebasic function of this particular encoding circuit and encoding schemeis to produce a series of pulses having a time displacement betweenpulses that corresponds to the sensor 12 output reading. Such a circuitand encoding scheme are fully described in U.S. Pat. Nos. 5,075,631 and5,077,527 both issued to Patriquin and commonly owned by the assignee ofthe present invention, the entire disclosures of both patents beingfully incorporated herein by reference. Reference to these patentsshould be made for a detailed explanation of the encoder circuit.Essentially, a ramp generator 150 is used to produce a voltage rampsignal 152. The ramp generator is conveniently realized in the form ofan integrator amplifier. The ramp signal 152 is input to one input 154of a comparator 156. The other input 158 to the comparator 156 isconnected to a multiplexer or signal switcher circuit 160 controlled bya timing control circuit 162 which can be realized with a counter. Thecontrol circuit 162 sequentially applies the +V_(ref), -V_(sensor) and-V_(ref) to the comparator input 158. The particular sequence onlyrequires that the sensor voltage be applied temporarily between the tworeference voltages. The comparator 156 output changes state when theramp voltage reaches the level of the applied signal to the othercomparator input. After the comparator changes state, the controlcircuit switches in the next signal to the comparator input 158 so thatthe comparator 158 output is a series of short pulses. The comparatoroutput 162 is connected to the LED/Driver circuit 54 (FIGS. 2 and 8.)

With reference to FIG. 7, I show a timing diagram for typical pulsesoutput from the encoder circuit of FIG. 6. In this case, the controller60 (FIG. 2) turns off the light source 56 after sufficient time haspassed (as at 170) to charge the storage capacitors 106,110 (FIG. 5.) Attime T1, the comparator 156 outputs a first pulse that has a leadingedge determined by the value of the +V_(ref) signal. The second pulsehas a leading edge at time T2 and this corresponds to the value of thesignal V_(sensor). The third pulse has a leading edge at time T3 that isdetermined by the value of the -V_(ref) signal. Because the ramp voltageV_(ramp) is a stable constant, and the reference voltages +V_(ref) and-V_(ref) are stable constants, the time delay between the leading edgesat T1 and T3 should be a fixed reference time. The time occurrence ofleading edge T2 which corresponds to the sensor data will vary with thesensor 12 resistance value and thus correspond to the fuelquantity/level in the tank. Thus, the ratio (T1-T2)/(T1-T3) defines thefractional immersion of the sensor 12 in the fuel.

It will be appreciated by those skilled in the art that in the specificexample described herein, leading edge timing is used for encoding.However, other timing sequences can be used just as conveniently.

With reference to FIG. 8, I show a simple circuit for interfacing theencoder 52 to the LED/Driver circuit 54. The V⁺ and V⁻ supplies areconnected to the corresponding terminals 116,120 from the storagecircuit in FIG. 5. An LED, or alternatively an array of LEDs, are seriesconnected with a switch 182. The switch 182 has a control input 184 thatis connected to the output 162 of the encoder comparator 156. Thus, theswitch 182 is pulsed on and off by the encoded pulses from thecomparator 156, thus correspondingly pulsing the LED 180 on and off. Alens 186 collimates the light from the LED 184 and transmits the lightpulses through a window 188 towards the hand held unit. The lens andwindow are, of course, optional depending on the particular application.

With reference to FIG. 6D I show another embodiment (with likecomponents being given like reference numerals followed by a prime') foran encoder circuit suitable for use with my invention. In this example,I still use an output comparator 156' to produce a series of pulseshaving a time domain displacement that corresponds to the sensor 12output dam. A control circuit 162' is used to switch sequentially to thecomparator (by means of a switching circuit 160') signals from a pair ofone shot timers 190,192. The first one shot produces a pulse edge at atime that corresponds to a reference resistor 140', and the second oneshot 192 produces a pulse edge that corresponds to the sensor 12resistance value. The first one shot may be armed from a signal producedby the energy storage circuit 46 (FIG. 5), and the second one shot armedby the pulse produced by the reference one shot. As in the otherdescribed embodiment, the comparator output 162' is a series of pulsesencoded with the sensor 12 reading.

As described hereinbefore, the hand held unit 34 includes appropriatecircuitry for transducing the encoded light received from the controlcircuit 40 by means of the detector circuit 58. The controller 60 can beeasily programmed to calculate the ratio (T1-T2)/(T1-T3) to determinethe fuel level and quantity.

Although not shown in detail in the Figures, the encoder circuit 52 mayinclude a second or more ramp generator(s) and associated circuitry togenerate a pulse series that encodes identification data such as thesensor 12 number, location, plane identifier and so on. Alternatively,the encoder could transmit one or more digital words (for example by useof a programmable ROM or other memory device) to the hand held unit 34before the sensor data is transmitted, with the digital words beingencoded data of interest to the ground crew.

While the invention has been shown and described with respect tospecific embodiments thereof, this is for the purpose of illustrationrather than limitation, and other variations and modifications of thespecific embodiments herein shown and described will be apparent tothose skilled in the art within the intended spirit and scope of theinvention as set forth in the appended claims.

I claim:
 1. Apparatus for remotely receiving sensor data from a liquidquantity sensor associated with a liquid container, comprising: sensorcontrol means coupled to the sensor for producing a firstelectromagnetic signal based on the sensor data, and a mobile remotecontrol means for receiving said first electromagnetic signal andproducing an output based on the sensor data; said sensor and sensorcontrol means being selectively powered in response to a signal fromsaid remote control means to produce said first electromagnetic signaland being substantially deenergized other than to produce said firstelectromagnetic signal.
 2. The apparatus of claim 1 wherein saidelectromagnetic signal is transmitted through free space to said remotecontrol means.
 3. The apparatus of claim 1 wherein said mobile remotecontrol means comprises a hand held unit.
 4. The apparatus of claim 1further comprising an aircraft fuel tank; wherein the sensor is disposedon the aircraft fuel tank, said sensor control means being disposed atthe fuel tank for transmitting said first electromagnetic signal to saidremote control means.
 5. Apparatus for remotely receiving sensor datafrom a fuel quantity sensor, comprising: an aircraft fuel tank; a fuelsensor disposed on the fuel tank for detecting fuel quantity therein;sensor control means disposed at the fuel tank and that is coupled tothe sensor for producing a first electromagnetic signal based on thesensor data; and a mobile remote control means for receiving said firstelectromagnetic signal and producing an output based on the sensor data;said sensor and sensor control means being selectively powered inresponse to a signal from said remote control means to produce saidfirst electromagnetic signal and being substantially deenergized otherthan to produce said first electromagnetic signal.
 6. The apparatus ofclaim 5 wherein said sensor control means and the sensor areelectrically energized for producing said first electromagnetic signal,and substantially deenergized other than when producing said firstelectromagnetic signal.
 7. The apparatus of claim 6 wherein said sensorcontrol means and sensor are electrically energized in response to asecond electromagnetic signal.
 8. The apparatus of claim 7 wherein saidremote control means transmits said second electromagnetic signal tosaid sensor control means.